Treatment of 7-amino-3-beta-D-ribofuranosylpyrazolo[4,3-d]pyrimidine (formycin) with alpha-acetoxyisobutyryl bromide followed by deprotection of the resulting trans-vicinal acetoxy bromides and hydrogenolysis of the separated bromohydrins gave 2'-deoxy-(23%) and 3'-deoxyformycin (32%) after complete deprotection and purification of their hydrochloride salts. An analogous sequence gave 3'-deoxytoyocamycin and/or 3'-deoxysangivamycin in approximately 80% yields from toyocamycin. Antiviral, antineoplastic, and antimetabolic effects were evaluated for the formycin compounds and 4-amino-7-beta-D-ribofuranosylpyrrolo[2,3-d]pyrimidine (tubercidin), its 5-cyano- (toyocamycin), and 5-carbamoyl-(sangivamycin) antibiotic congeners in comparison with their 2'-deoxy, 3'-deoxy, and arabino analogues. In all cases, the modified-sugar compounds were less cytotoxic than the parent antibiotics. The majority also exhibited lower antiviral potency. However, the xylo-tubercidin analogue retained potent antiherpes 1 and 2 activity with decreased cytotoxicity. Labeled metabolite studies suggested that effects of these compounds on RNA and/or protein synthesis might be more significant than interference with DNA synthesis.